Memory performance is a well known limitation to computer system performance. Ideally, the memory performance would match or exceed processor performance, i.e., a memory cycle time would be less than one processor clock cycle. This is almost never the case and, so, the memory is a system bottleneck. A state of the art high speed microprocessor may be based on a 200 MegaHertz (MHZ) clock with a 5 nanosecond (ns) clock period. A high performance DRAM may have a 60 ns access time, which falls far short of processor performance.
This system bottleneck is exacerbated by the rise in popularity of multimedia applications. Multimedia applications demand several times more bandwidth for main memory or frame-buffer memory than computational intensive tasks; tasks such as spread sheet analysis programs or, other Input/output (I/O) intensive applications such as word processing or printing.
Extended Data Out (EDO) and Synchronous DRAMs (SDRAMs) were developed to improve bandwidth. However, SDRAMs and EDO RAMs still do not match processor performance and, therefore, still limit system performance. Consequently, as faster microprocessors are developed for multimedia processing and high performance systems, faster memory architecture is being developed to bridge the memory/processor performance gap, e.g., wide I/O DRAMs.
While the problem of matching memory speed to processor performance is being addressed, memory reliability in such a high performance system has not. Traditional Error Checking Code (ECC) schemes are not easily adapted to such a high bandwidth/performance memory chip organization. Typical ECC schemes include parity checking and Single Error Correction--Double Error Detection (SEC/DED) codes.
Parity is preferred for narrow data words, i.e., eight or sixteen bits because it requires only one extra bit per byte. However, parity only indicates that one or more of the bits in the byte/word is in error.
SEC/DED codes are more reliable than parity, but for short data words (one or two bytes) require several check bits per word, and are, therefore, seldom used in such applications. SEC/DED is more efficient for a wider data word, i.e., 64 bits or greater, because for a wide data word (several bytes) the check bit/byte ratio may be reduced to less than one.
However, adding ECC to a memory system requires adding logic and extra memory chips for the six or more check bits. This extra memory may be in the form of extra memory chips or, special memory chips with nine or eighteen data input/outputs (I/Os).
Thus, there is a need for high bandwidth DRAM chips that lend themselves to ECC applications for high performance memory architectures.